Silicon surface preparation

ABSTRACT

Methods are provided for producing a pristine hydrogen-terminated silicon wafer surface with high stability against oxidation. The silicon wafer is treated with high purity, heated dilute hydrofluoric acid with anionic surfactant, rinsed in-situ with ultrapure water at room temperature, and dried. Alternatively, the silicon wafer is treated with dilute hydrofluoric acid, rinsed with hydrogen gasified water, and dried. The silicon wafer produced by the method is stable in a normal clean room environment for greater than 3 days and has been demonstrated to last without significant oxide regrowth for greater than 8 days.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.11/210,441, filed Aug. 23, 2005, which is incorporated herein byreference in its entirety.

This application is also related to U.S. Pat. No. 6,620,743 to Pagliaroet al., filed Mar. 26, 2001.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to methods for preparing a clean and stablesilicon surface.

2. Description of the Related Art

Clean semiconductor surfaces are a key factor in preparing integratedcircuits with high yields. There are two major types of contaminationthat occur on semiconductor surfaces: films and particulates.Particulates are materials that have readily defined boundaries, whilefilms (for example, native oxide on a bare silicon surface) are layersof material on the surface of the wafer.

It is important to minimize or eliminate both films and particulates onthe surface of the silicon wafer in order to optimize integrated circuityields. Prior to epitaxial deposition and diffusion processes on baresilicon surfaces, particularly for processes conducted at less thanabout 850° C., it is important to have a clean silicon surface.

Particulates and films may be removed through cleaning. The standardcleaning method often involves one or more forms of an RCA cleaningprocedure. The RCA Standard-Clean-1 (SC-1) procedure uses a mixture ofhydrogen peroxide, ammonium hydroxide, and water heated to a temperatureof about 70° C. The SC-1 procedure dissolves films and removes Group Iand II metals. The Group I and II metals are removed through complexingwith the reagents in the SC-1 solution.

The RCA Standard-Clean-2 (SC-2) procedure utilizes a mixture of hydrogenperoxide, hydrochloric acid, and water heated to a temperature of about70° C. The SC-2 procedure removes the metals that are not removed by theSC-1 procedure. If an oxide-free surface is required, the silicon waferis dipped into an aqueous solution of hydrofluoric acid to etch away thenative oxide layer and, theoretically, obtain hydrogen termination.There are a large number of variations on RCA clean and hydrofluoricacid dips.

After cleaning, wafers are typically stored for a period of time beforefurther processing. Silicon-fluorine and silicon-carbon bonds are oftenobserved on the silicon surface after cleaning. The fluorine and carboncontamination on the surface may be detrimental to the thermal budgetand/or the quality of the layer to be grown or deposited on the surfaceof the wafer.

If the silicon wafer is dipped in hydrofluoric acid as the last cleaningstep (also known as an “HF last” step), the surface of the silicon istypically terminated mostly with a monolayer of hydrogen, largely Si—Hbonds. The hydrogen-terminated surface prevents oxidation better thanwithout any termination. However, the surface of a silicon wafer afteran HF last treatment normally starts to reoxidize within about 20minutes after the original oxide layer was removed, quickly forming anew 5 Å to 7 Å thick oxide layer on the surface of the silicon wafer.Even with the best cleaning processes currently known, a layer of nativeoxide forms within 48 hours, and, often the wafers cannot be furtherprocessed within that time. This will mandate a new HF dip or in situvapor clean if an oxide-free surface is required for the next processstep.

In an HF last, when the oxide layer is removed from the surface with ahydrofluoric acid solution as the final step in the cleaning procedure,the wafer surface has a tendency to have high levels of particles dueto: 1) exposure to contaminants in the solution; 2) exposure to air atthe air/liquid interface; 3) deposition of particles during the dryingprocess; and 4) exposure to air during the time between the drying stepand the time that the silicon wafer is placed in an inert environment.

U.S. Pat. No. 6,620,743 to Pagliaro et al. teaches a method for forminga stable, oxide-free silicon surface. The '743 patent teaches anoptimized APM clean, followed by a dilute HF etch, then an in siturinse, and a dry only spin dry. The method taught by the '743 patentachieve silicon surfaces with a desirable level of stability, but isdisadvantageous in certain respects. For example, the method requiresshort time intervals between its processes, is time-consuming, andutilizes expensive equipment.

Accordingly, there is a need for improved methods of preparing a cleanand stable silicon surface.

SUMMARY OF THE INVENTION

In accordance with one aspect of the invention, a method is provided forpreparing a silicon surface. The method includes treating the siliconsurface with dilute hydrofluoric acid comprising anionic surfactant, andthen rinsing and drying the silicon surface.

In the illustrated embodiments, a dilute HF dip is carried out thatincludes 20 to 200 ppm of anionic surfactant. The dilute HF solution isthen rinsed away in-situ, and the substrate (e.g., silicon wafer) isdried. Drying is carried out using heated, ionized, high purity purgegas (i.e., N₂, Ar) or isopropyl alcohol. Advantageously, the treatingand rinsing steps employ ultrapure water with a resistivity at 25° C. ofgreater than 16 MΩ-cm and less than 10 ppb total oxidizable carbon, lessthan 10 ppb dissolved silica, and less than 500 ppb dissolved oxygen.The treating, rinsing, and drying are all carried out in a single vesselprocessor. This process has been shown to demonstrate a particle removalefficiency of 35%-55%, with native oxide growth limited to less than 1 Åafter exposure to clean room air for more than about 5 days. Notably,these results are achieved without the use of an APM clean.

In accordance with another aspect of the invention, a method is providedfor forming an integrated circuit. The method includes treating asurface of the integrated circuit with aqueous hydrofluoric acidcomprising anionic surfactant. In an illustrated embodiment, the anionicsurfactant has a concentration of 20 to 200 ppm and is configured tocharge the zeta potential of particles on the surface of the integratedcircuit negatively.

In accordance with another aspect of the invention, a method is providedfor preparing a silicon surface by treating the surface with dilutehydrofluoric acid, rinsing the surface with hydrogen gasified water, anddrying the surface. In the illustrated embodiments, the hydrogengasified water has a dissolved hydrogen concentration of 1.2 to 1.6 ppm.Rinsing is carried out for approximately 2-3 minutes and includesmegasonic energy at 900 to 1000 kHz. This process has been shown todemonstrate a particle removal efficiency of greater than 95%, withnative oxide growth limited to less than 1 Å after exposure to cleanroom air for more than about 5 days.

In accordance with another aspect of the invention, a method is providedfor forming an integrated circuit, which method includes exposing asurface for formation of the integrated circuit to hydrogen gasifiedwater.

In accordance with another aspect of the invention, a method is providedfor preparing water for semiconductor processing. The method includeshydrogen gasification of the water.

In accordance with another aspect of the invention, a method is providedwhereby water is exposed to ultraviolet radiation, filtered, degasifiedand gasified with hydrogen.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram illustrating a method of preparing a siliconsurface according to embodiments of the present invention.

FIG. 2 is a block diagram illustrating a method of preparing a siliconsurface according to other embodiments of the present invention.

FIG. 3 is a block diagram illustrating a method of preparing water forsemiconductor processing according to embodiments of the presentinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Embodiments of the method of the present invention provide a method forproducing silicon wafers having pristine, hydrogen-terminated stablesurfaces. Oxide re-growth on the silicon surface produced with someembodiments has been shown to be inhibited in a clean room environmentfor over 5 days, and in some cases for more than 8 days. As discussedabove, the method described in the '743 patent achieves stabilityagainst significant native oxide growth for up to 8 days. However, thatmethod demands significant capital, in the form of chemicals, equipment,and power consumption. Another significant capital expenditure is theadditional cycle time consumed by the APM clean during the production ofa silicon surface. Furthermore, chemicals used in the APM clean may havethe potential to introduce safety or environmental hazards. Moreover, asmentioned above, the APM clean risks contaminating the silicon surfacewith metals. Perhaps most importantly, the APM clean disadvantageouslyconsumes silicon that is part of the substrate. Known methods for APMcleans typically consume approximately 3-5 Å of a silicon surface. Aloss of this thickness of silicon becomes increasingly problematic asfeatures and components of integrated circuit devices become smaller.

Certain embodiments of the present invention achieve stability withoutthe need for the costly and time-consuming step of an APM clean(although other embodiments provide for the optional use of an APMclean). The preferred embodiments therefore provide a method ofpreparing a stable silicon surface in a more time- and cost-efficientmanner than known production methods. Although the embodiments of themethod of the invention are described in the context of cleaning a baresilicon wafer, it is to be understood that the preferred embodimentshave broad applicability to cleaning a wide range of surfaces.

FIG. 1 shows the steps in an embodiment of the present invention. Thefirst step 10 is an optional cleaning of the wafer with a mixture ofammonium hydroxide and hydrogen peroxide, known in the industry as anammonium hydroxide/peroxide mixture (APM). The second step 20 is atreatment with dilute hydrofluoric acid with an anionic surfactant. Thethird step 30 is an in-situ rinse, and the fourth step 40 is a substratedrying step. Each of these steps will be described in more detail below.

Ammonium Hydroxide/Hydrogen Peroxide Cleaning

Step 10 of FIG. 1 involves optionally cleaning the silicon wafer with anammonium hydroxide/hydrogen peroxide mixture (APM). The ammoniumhydroxide/hydrogen peroxide cleaning step 10 of the preferred embodimentuses a solution of 800 mL to 1,000 mL of 30% hydrogen peroxide, 300 mLto 600 mL of 29% ammonium hydroxide and 11 gallons (41 L) of water.Thus, the total bath concentration is preferably 0.50% to 0.80% byvolume ammonium hydroxide, more preferably 0.58% to 0.73% ammoniumhydroxide. The total bath concentration is preferably between about0.10% to 0.50% hydrogen peroxide, more preferably about 0.21% to 0.42%hydrogen peroxide. The solution is preferably maintained at atemperature of about 20° C. to 50° C., more preferably 30° C.-40° C. andthe wafer is maintained in the solution for about 5 minutes to 15minutes. The APM solution of step 10 of FIG. 1 is similar to the SC-1solution of the RCA cleaning process.

The APM cleaning step 10 removes particles, surface defects, and Group Iand Group II metals from the silicon wafer in the process of growing achemical oxide. The APM cleaning may be done in an open vessel bath orother suitable vessel. Open vessel baths are commercially available. TheSuperior Automation Recirculating Bath, commercially available fromSuperior Automation, San Jose, Calif. is an exemplary open vessel bathwhich is suitable for use in the optional APM cleaning step 10. Otheropen vessel baths are suitable for the optional APM cleaning step 10.Furthermore, the optional APM cleaning step 10 is not limited to openvessel baths.

Dilute Hydrofluoric Acid Treatment

The dilute hydrofluoric (dHF) acid treatment step 20 of FIG. 1 may beperformed after the APM cleaning step 10 or as a first step in thepreparation of a silicon surface.

The dilute hydrofluoric acid for the dilute hydrofluoric acid treatmentstep 20 preferably has a concentration of approximately 0.25% to 1.0% byvolume (vol. %) hydrogen fluoride HF, more preferably about 0.25 wt. %to 0.5 wt. % HF. Use of a dilute hydrofluoric acid for the treatmentstep 20 minimizes contamination on the surface of the silicon wafer. Thedilute hydrofluoric acid is preferably heated to a temperature of about30° C. to 50° C., more preferably at about 40° C., to minimize particlesand to enhance hydrogen termination on the surface of the silicon wafer.The heated dilute hydrofluoric acid treatment also provides for uniformoxide etch rates on the entire surface of the silicon wafer. The siliconwafer is preferably exposed to the dilute hydrofluoric acid treatmentfor a time period of between about 20 seconds and 2 minutes, morepreferably for a time period of between about 40 seconds and 60 seconds,and most preferably for a time period of approximately 60 seconds. Thus,the silicon wafer can be treated with dilute hydrofluoric acid having aconcentration of approximately 0.5 vol. % hydrogen fluoride at atemperature of approximately 40° C. for approximately 60 seconds.

The dilute hydrofluoric acid used in treatment step 20 of theillustrated embodiment includes a surfactant. Preferably, the surfactantis an anionic surfactant, which charges the zeta potential of particlessuspended in the bath chemistry and on the silicon surface negatively.The negative charging of those particles serves to dissociate theparticles from the similarly charged silicon surface. Thus, the anionicsurfactant aids in removing contaminating particles from the surface ofthe silicon by changing their zeta potential to a negative charge andcausing a repulsive force to liberate them from the negatively chargedsilicon surface. Anionic surfactant is particularly helpful in removingpolymeric particles such as PEEK and Teflon from hydrophobic surfaces.Furthermore, anionic surfactants advantageously prevent metal iondeposition. In addition, if the anionic surfactants are used in abuffered hydrofluoric acid solution, the anionic surfactants can preventmicroroughness generation. In the preferred embodiment, the anionicsurfactant has a concentration of 20 to 200 ppm. The surfactant may bean organic surfactant, such as hydrocarbonic sulfate orperfluorocarbonate.

The ultrapure water used to form the dilute hydrofluoric acid has highresistivity, indicating that the metals levels are low. By using waterhaving high resistivity to form the dilute hydrofluoric acid, thequantity of metals which are deposited on the silicon wafer during thedilute hydrofluoric acid treatment step 20 is minimized. The water usedto form the dilute hydrofluoric acid has a resistivity greater thanabout 15 megaohms-cm (MΩ-cm) at a temperature of 25° C., and mostpreferably a resistivity of at least about 17.5 MΩ-cm. The totaloxidizable carbon (TOC) and the dissolved silica are also preferablyminimized to levels of less than 10 ppb (parts per billion).

Several water treatments are preferably employed to achieve thesestringent levels of water purification. These treatments havesubstantial overlap with the methods illustrated in FIG. 3, with thesignificant exception that the treatments for the embodimentsillustrated in FIG. 1 do not include gasifying the water with dissolvedhydrogen. As such, description of the method of water treatment for usein all treatment and rinse steps is deferred until the methodsillustrated by FIG. 3 are described.

The hydrofluoric acid used to form the dilute hydrofluoric acid ispreferably gigabit grade (on the order of parts per trillion impurities)hydrofluoric acid with low levels of particles and dissolved metals,commercially available as Part No. 107101 in a 49% solution from AlamedaChemical of Tempe, Ariz., (480) 785-4685.

In the preferred embodiments, a high purity nitrogen purge curtain isemployed at the air liquid interface during both the dilute hydrofluoricacid treatment step 20 and the rinse step 30 discussed below. The highpurity nitrogen is filtered through a filter, which removes particleslarger than 0.003 μm at the point of use. Ionizing the nitrogen beforethe nitrogen contacts the silicon wafer minimizes particles. The highpurity nitrogen enhances particle neutrality and stable surfacetermination on the silicon wafer.

Rinse

After the silicon wafer is treated with dilute hydrofluoric acid in step20, the silicon wafer is rinsed with ultrapure water for maximumhydrogen passivation of the treated silicon surface in the rinse step 30of FIG. 1. The ultrapure water which is used for the rinse 30 desirablyhas the same purity as the ultrapure water which is used to form thedilute hydrofluoric acid to maintain stable hydrogen termination andparticle neutrality. The treated silicon wafer is preferably rinsed withultrapure water for a time period sufficient to remove all HF acid andparticles from the previous etch step. The specific period of timedepends upon the volume of the vessel used for treatment and the flowrate of the rinse water.

Preferably, the rinse is an in-situ rinse. Rinsing the silicon waferin-situ in the vessel used for step 20 minimizes the amount ofcontamination, including reoxidation. Further, an in-situ rinseeliminates the step of transferring the silicon wafer to a rinse bath.Contamination of the silicon wafer would tend to occur during thetransfer to the rinse bath. In the illustrated embodiment, the in-siturinse is conducted at approximately room temperature (typically 20°C.-25° C., or about 23° C.). The in situ rinse is preferably acascade/overflow rinse in either a single vessel processor or in arecirculating and filtered etch bath. Single vessel processors typicallyinclude integrated drying capability and single pass etch chemistry, incontrast to the recirculating and filtering etch bath.

An exemplary rinse step 30 involves an in-situ rinse in the vessel usedfor step 20 with ultrapure water at room temperature for approximately15 minutes.

Drying

After the silicon wafer is rinsed with ultrapure water, the siliconwafer 50 is dried in the drying step 40 of FIG. 1. Although a variety ofdrying apparatuses are suitable for the drying step 40, the Rhetech480ST is an exemplary spin/rinse dryer, commercially available fromRhetech, Inc. of Coopersburg, Pa. In an exemplary embodiment, thesilicon wafer may be transferred to the spin/rinse dryer after rinsingin the in-situ rinse step 30. In the spin-only dry step 40, the siliconwafers are spun dry while hot, ionized nitrogen is flowed into the dryerat a rate of between about 15 slm and 25 slm. The hot nitrogen gaspreferably at a temperature of 60° C. to 80° C., more preferably at atemperature of 60° C. to 80° C., and most preferably at a temperature ofabout 70° C. The dry cycle is carried out at 400 rpm to 600 rpm, withoutusing the rinse cycle of the machine. The nitrogen stream is passedthrough a filter, which removes particles larger than 0.003 μm beforeentering the dryer. Thus, in an exemplary embodiment, the silicon waferis dried in a drying step 40 at 500 rpm for 240 seconds for a baresilicon wafer or 480 seconds for a patterned silicon wafer with theheater on and antistat (ionization) on.

Alternatively, the drying step 40 may employ an isopropyl alcohol (IPA)based technique. Examples of IPA-based drying tools that would besuitable for embodiments of the present invention are the APET RD andthe AP&S AeroSonic rinsing and drying tools. In some embodiments, thesilicon wafer may be exposed to IPA as part of the treatment processprior to a N₂ dry step; in some embodiments the IPA treatment itselfserves to dry the wafers.

In some embodiments, the drying step 40 may be carried out in the samevessel in which the treatment step 20 and the rinsing step 30 werecarried out. Such embodiments advantageously avoid the necessity of atransfer step and the attending risk of surface contamination. Thus,embodiments of the present invention make possible the use of singlevessel processors to carry out the treatment step 20, rinsing step 30,and drying step 40. Both of the aforementioned IPA-based drying toolsare considered examples of single vessel processors when the availableHF injection option is utilized for the initial etch step.

The drying step 40 is carried out until the silicon wafers are dry. Thedrying step 40 is preferably also optimized to ensure near particleneutrality (i.e. adds fewer than 0.032 particles/cm² having a sizelarger than 0.12 μm to the silicon surface) and stable surfacetermination on the silicon wafer.

The embodiments illustrated in FIG. 1 are simple and economical means ofachieving a silicon wafer having high surface stability. All of theinstrumentation used in the method are commercially available, and theprocess conditions are typically adaptable to most wet wafer cleaningprocessors.

Silicon wafers that are prepared by the embodiments illustrated by FIG.1 have surfaces that are stable against oxidation for greater than 5days, more preferably greater than 6 days, and most preferably greaterthan 8 days. A hydrogen-terminated silicon surface is considered“stable” against oxidation, as used herein, if the surface has anaverage thickness of less than 1 Å oxide on the surface when the siliconsurface is stored in air in a clean room environment. The optimizedconditions disclosed herein have showed as low as 0.1 Å oxide re-growthafter 8 days.

In addition, the embodiments illustrated by FIG. 1 demonstrate aParticle Removal Efficiency (PRE) of greater than 25%, and preferablybetween 35% and 55%. Without being limited by a theory, it is believedthat the stability of the hydrogen-terminated silicon surface producedwith the preferred embodiments is aided by minimizing the number ofparticles added to the surface of the silicon surface during the stepsof the method illustrated in FIG. 1. The PRE measures the Particle Counton the silicon surface after the drying step 40 compared to before thetreatment step 20, according to the formula:

PRE=100*(PC_(Before Treating)−PC_(After Drying))/(PC_(Before Treating))

where “PRE” signifies Particle Removal Efficiency of the entiretreatment/rinse/dry process and “PC” signifies Particle Count. ParticleCounts for the above disclosed PREs were measured using a TencorSurfscan® 6200 or SP-1 particle counter, available commercially fromKLA-Tencor of San Jose, Calif.

It was noted above that prior to epitaxial deposition and diffusionprocesses on bare silicon surfaces, particularly for processes conductedat less than about 850° C., it is important to have a pristine siliconsurface. Thus, the method illustrated in FIG. 1 is useful as apreparation of a silicon surface for later forming an epitaxial layer onthe silicon surface, particularly for low temperature epitaxy at lessthan 850° C.

Advantages of the method illustrated in FIG. 1 include:

-   -   1. The processing is done at low temperatures;    -   2. The cost of the equipment and chemicals is low;    -   3. The method is readily accepted by customers;    -   4. The method can be employed using a wide range of commercially        available equipment;    -   5. The etch chemistry is simple; and    -   6. The method is safe and produces a minimum of environmentally        hazardous waste products.

FIG. 2 shows the steps in a second embodiment of the present invention.The first step 22 is a treatment with hydrofluoric acid diluted inhydrogen gasified water. The second step 32 is an in-situ rinse withhydrogen gasified water and megasonic energy. The third step 42 is asubstrate drying step, which can be substantially the same as the dryingstep 40 illustrated in FIG. 1 and described above. The treatment step 22and rinse step 32 will be described in more detail below. Because thedrying step 42 can be substantially the same as the drying step 40described above, a detailed description of that step is omitted below.

Significantly, the embodiments illustrated in FIG. 2 also avoid thenecessity of an APM clean. Although the APM cleaning step 10 may beemployed to obtain a silicon wafer having high stability according tocertain embodiments of the method of the present invention, the APMclean has certain drawbacks that may make it desirable to omit the APMcleaning step 10. For example, the APM cleaning step 10 demands capitalin the form of chemicals, equipment, and power consumption. Anothersignificant capital expenditure in the APM cleaning step 10 is theadditional cycle time it requires during the production of a siliconsurface. Furthermore, the ammonium hydroxide, hydrogen peroxide, and theby-products associated with the APM cleaning step 10 may have thepotential to introduce safety or environmental hazards. Moreover, theAPM cleaning step 10 risks contaminating the silicon surface withfluorine, carbon, metals, or other contaminants that may be present inthe cleaning solution. Such contamination on the surface may bedetrimental to the thermal budget and/or the quality of the layer to begrown or deposited on the surface of the wafer.

Perhaps most importantly, the APM cleaning step 10 disadvantageouslyconsumes silicon that is part of the substrate. Known methods for APMcleans typically consume approximately 3-5 Å of silicon a siliconsurface. A loss of this thickness of silicon becomes increasinglyproblematic as features and components of integrated circuit devicesbecome smaller. As such, embodiments of the present invention make itpossible to omit the APM cleaning step 10, while still achieving thestability and particle removal efficiency of known methods for preparinga silicon surface.

Dilute Hydrofluoric Acid Treatment

The dilute hydrofluoric acid treatment step 22 according to the methodillustrated in FIG. 2 is similar to step 20 of the method illustrated inFIG. 1, except that step 22 does not include anionic surfactant. In someembodiments, step 22 preferably includes diluting the hydrofluoric acidin hydrogen-gasified water, or otherwise hydrogenating the aqueous HFsolution. The hydrogenation of the HF solution in the treatment step 22advantageously creates a surplus of hydrogen radicals (H+) to enhanceoptimum Si—H terminations of the silicon surface during the dissociationand removal of the native oxide. Hydrogen gasified water can be preparedaccording to the method illustrated in FIG. 3, disclosed below andincluded in the preparation of the dilute HF solution. However, theadvantageous stability and hydrogen termination can be obtained even inembodiments that do not utilize hydrogen gasified water in treatmentstep 22, but do employ hydrogen gasified water in rinse step 32.

Rinse

After the silicon wafer is treated with dilute hydrofluoric acid in step22, the silicon wafer is rinsed in-situ with hydrogen gasified water inthe in-situ rinse step 32 of FIG. 2. The hydrogen gasified water usedfor the in-situ rinse 32 desirably has a dissolved hydrogenconcentration of 1.2 to 1.6 ppm, and is prepared according to the methodillustrated in FIG. 3, which is described below. The use of hydrogengasified water advantageously creates a surplus of hydrogen radicals(H+) to enhance optimum Si—H terminations during the rinse step andcharges the zeta potential of particles suspended in the chemistry ofthe bath positively, deterring particles from adhering to the siliconsurface. Thus, the hydrogen gasified water aids in forming ormaintaining the hydrogen termination of the silicon surface, which aidsin preventing contaminating particles from adhering to the surface ofthe silicon.

Preferably, megasonic energy is applied to the silicon surface duringthe rinse step. Supplementing the rinse step 32 with megasonic energysupplies a catalyst to the process of removing the particles on thesilicon surface and to the beneficial hydrogen termination of thesilicon surface. Megasonic energy is applied to the silicon surface at afrequency of 800 to 1200 kHz, and preferably at a frequency of 900 to1000 kHz. The use of megasonic energy substantially reduces the requiredrinse time. With optimal conditions, this method has been used toprepare a silicon surface with a PRE of greater than 95% for the entiredHF treatment/ in situ rinse/ dry cycle with a rinse step 32 lastingonly 2-3 minutes. An example of a transducer that can be utilized inthis embodiment is available commercially from ProSys of Campbell,Calif. The transducer plate(s) of the megasonic system are mounted tothe processing vessel to optimize the energy distribution uniformityacross the silicon surface. This permits the H-radicals, which have avery short lifetime, to terminate the dangling silicon bonds.

Silicon wafers that are prepared by the embodiments illustrated by FIG.2 have surfaces that are stable against oxidation for greater than 5days, more preferably greater than 6 days, and most preferably greaterthan 8 days. In addition, the embodiments illustrated by FIG. 2preferably have a Particle Removal Efficiency (PRE) of greater than 25%,and more preferably greater than 85%, and have demonstrated PRE ofgreater than 95% with the combination of megasonic energy and hydrogengasification.

The method illustrated in FIG. 2 shares the primary advantages of themethod illustrated in FIG. 1, which were described above.

FIG. 3 shows the steps in an embodiment of the present inventiondirected to preparing water for semiconductor processing. The first step50 in that process is passing the water through resin beds in order tosoften the water and remove solvents. The second step 60 is exposing thewater to ultraviolet radiation to kill bacteria and fungi. The thirdstep 70 is filtering the water to remove the particles formed in killingthe bacteria and fungi, as well as other undesirable residual particles.The fourth step 80 is degasifying the water to minimize, inter alia,dissolved oxygen levels. The fifth step 90 is gasifying the water withdissolved hydrogen. The sixth step 100 is ultra-filtration, whichutilizes a combination filter with positive and negative zeta chargedmedia. Each of these steps will be described in greater detail below.This method may be undertaken in conjunction with or in preparation forthe methods illustrated in FIG. 2.

In a preferred embodiment, municipal water is first softened by passingit though water softening resins (e.g., sodium zeolite cation resins)removing calcium and magnesium. A downstream reverse osmosis unit, suchas the Filmtec⊥ BW30-4040, removes about 98% of total dissolvedsolvents. The water is then subjected to a primary demineralizer,preferably comprising a mixed bed of ion exchange resins. Exemplarypolystyrene beads are available from Rohm & Haas in 40% anion resin/50%cation resin mix. A downstream resin trap (1 μm filter) leads to astorage tank lined with polyethylene, polyvinylidenefluoride (PVDF) orother suitable materials to avoid contamination. A 2,000 gallon tank isemployed in the preferred embodiment.

Downstream of the storage tank, water is constantly looped through aplurality of further treatments to ensure purity prior to use. Thefurther treatments include exposure to a first ultraviolet (UV) source,preferably comprising a 254 μm ultraviolet (UV), available commerciallyas part number 1H-8L TOC Reduction unit from WEDECO Ideal Horizons, Inc.of Poultney, Vt. Filters remove particles down to about 0.2 μm, and amixed bed of ion exchange resins (50/50 mix of anion and cationexchangers), with an attendant resin trap filter separate the first UVsource from a second UV source. In the preferred embodiments, the secondUV source comprises a 185 nm narrow band UV lamp, commercially availablefrom Ideal Horizons, Inc. of Poultney, Vt. as part number 1H-4L TOCReduction unit. Treatment with ultraviolet light kills bacteria andfungi in the water. The particles formed by killing the bacteria andfungi are removed in other treatment steps. Another 0.2 μm filterdesirably removes particles downstream of the second UV source.

The water purification system preferably also includes a plurality ofmonitors. In the preferred embodiment, the monitors includes aresistivity monitor (e.g., 200CR Resistivity Monitor, available fromThornton, Inc. of Waltham, Mass.) a pH monitor (e.g., part number63221-1, also available from Thornton, Inc. of Waltham, Mass.) a totaloxidizable carbon (TOC) analyzer (e.g., model A-1000 TOC AnalysisSystem, available from Anatel Corp. of Boulder, Colo.) and a particlecounter, also available from Anatel Corp.

Another preferred treatment involves removing dissolved oxygen from theultrapure water to a level of 200 ppb or less. The dissolved oxygen isremoved with a degasification module, such as the Liqui-Cel type G333,commercially available from Celgard of Charlotte, N.C. The water ispreferably also filtered through zeta charged (+ and/or −) pointof-use-filters to neutralize any particles in the water so that particleretention on the filters is maximized. The piping and as much of therest of the purification system as possible are made of PVDF(polyvinylidenefluoride) to minimize contamination.

At this point, the water could be used for the method illustrated inFIG. 1, described above. In order to prepare the water for the methodillustrated in FIG. 2, the water must is then gasified with hydrogen. Anexemplary gasification module is available commercially under the tradename KHOW® SYSTEM from Kurita Water Industries Ltd. of Tokyo, Japan.Preferably, the water after hydrogen gasification has a dissolvedhydrogen concentration of 1.2 to 1.6 ppm. Hydrogen gasified water isalso known as “functional water.”

Various modifications and alterations of this invention will be apparentto those skilled in the art without departing from the scope and spiritof this invention. It is to be understood that the invention is notlimited to the embodiments disclosed herein, and that the claims shouldbe interpreted as broadly as the prior art allows.

1. A method of preparing a silicon surface, comprising: treating thesilicon surface with dilute hydrofluoric acid in a treatment vessel;in-situ rinsing the silicon surface in the treatment vessel withhydrogen gasified water after treating, wherein the silicon surface isnot exposed to air between treating and rinsing; and drying the siliconsurface after rinsing.
 2. The method of claim 1, wherein the siliconsurface comprises a hydrogen-terminated silicon surface after treating,rinsing and drying.
 3. The method of claim 1, wherein the rinsingcomprises applying megasonic energy to the water.
 4. The method of claim3, wherein the megasonic energy comprises vibrations at a frequency ofapproximately 800 to 1200 kHz.
 5. The method of claim 3, wherein therinsing is conducted for approximately 2 to 3 minutes.
 6. The method ofclaim 5, wherein treating, rinsing and drying has a particle removalefficiency of greater than 95%.
 7. The method of claim 1, wherein thedilute hydrofluoric acid comprises hydrogen gasified water.
 8. Themethod of claim 1, wherein the hydrogen gasified water is configured topositively charge the zeta potential of particles on the siliconsurface.
 9. The method of claim 1, wherein the drying comprisesemploying isopropyl alcohol.
 10. The method of claim 1, wherein thetreating, rinsing, and drying are performed in a single vesselprocessor.
 11. The method of claim 1, wherein the treated, rinsed anddried silicon surface grows a native oxide of less than 1 Å afterexposure to air for more than about 6 days.
 12. The method of claim 11,wherein the treated, rinsed and dried silicon surface grows a nativeoxide of less than 1 Å after exposure to air for more than about 8 days.13. The method of claim 1, further comprising: forming an epitaxiallayer on the silicon surface; and forming an integrated circuit.
 14. Amethod of forming an integrated circuit, comprising exposing a surfaceof a substrate for formation of the integrated circuit to hydrogengasified water after treating the substrate surface with dilutehydrofluoric acid, wherein treating the surface and exposing the surfaceare conducted in the same vessel without subjecting the surface to airbetween treating and exposing.
 15. The method of claim 14, whereinexposing the surface to hydrogen gasified water comprises rinsing thesurface with hydrogen gasified water.
 16. The method of claim 14,wherein the surface comprises a hydrogen-terminated silicon surface. 17.The method of claim 14, wherein the exposing comprises exposing thesilicon surface to megasonic energy at a frequency of approximately 900to 1000 kHz.
 18. The method of claim 17, wherein the rinsing isconducted for approximately 2 to 3 minutes.
 19. The method of claim 14,wherein the surface is not cleaned with a solution comprising ammoniumhydroxide, hydrogen peroxide, and water.
 20. The method of claim 14,further comprising exposing the surface to isopropyl alcohol.
 21. Amethod of treating a silicon surface, comprising: treating the siliconsurface with dilute hydrofluoric acid; rinsing the silicon surface withhydrogen gasified water after treating, wherein the silicon surface isnot exposed to air between treating and rinsing; and drying the siliconsurface after rinsing.
 22. The method of claim 21, wherein rinsing isconducted for approximately 2 to 3 minutes.
 23. The method of claim 21,wherein the hydrogen gasified water has a dissolved hydrogenconcentration of approximately 1.2 to 1.6 ppm.
 24. The method of claim21, wherein treating, rinsing, and drying are performed in a singlevessel processor.